Why Do Cars Have Pneumatic Tires?
Pneumatic tires help to cushion the vehicle against bumps, ruts and other inequalities in the road surface. This leveling effect of the tire is achieved because of the compressed air which is inside the rubber casing. Until about 1900 all road vehicles were fitted with solid tires. These were very unsatisfactory. When a tire struck an obstacle on the road surface the shock often damaged the vehicle’s mechanism permanently.
In any case the effect was most uncomfortable for the passengers, and the roads then had holes and bumps which made driving over them dangerous. The adaptability of pneumatic tires gives them a firmer grip on the road and enables the driver to steer, change speed and go round corners more safely.
The main parts of a modern pneumatic tire are its body, tread and sidewalls, and beads. The body is made of layers of rubberized fabric, called plies that give the tire strength and flexibility. The fabric is made of rayon, nylon, or polyester cord. Covering the plies are sidewalls and tread of chemically treated rubber. The sidewalls form the outer walls of the tire. The tread is a thick hoop of rubber that comes into direct contact with road surfaces.
To improve its traction, the tread has patterns of deep and shallow grooves and channels, depending on the intended use, and also may have protruding metal studs for icy or snowy conditions. The pneumatic rubber tire uses rubber and enclosed air to reduce vibration and improve traction.
The first patent for a pneumatic tire was issued to Robert William Thomson in England in 1845 for a hollow leather tire filled with air. Although a set of Thomson’s “Aerial Wheels” ran for 1,200 miles on an English brougham, the same inventor’s solid-rubber tires were more popular; and thus, for almost half a century, air-filled tires were forgotten. The growing popularity of the bicycle in the late 19th century revived interest in tire design, and in 1888 John Boyd Dunlop, a veterinary surgeon of Belfast, obtained patents on a pneumatic tire for bicycles.
Pneumatic tires were first applied to motor vehicles by the French rubber manufacturer Michelin & Cie. For more than 60 years, pneumatic tires had inner tubes to contain the compressed air and outer casings to protect the inner tubes and provide traction. In the 1950s, however, tubeless tires reinforced by alternating plies, or layers, of cord became standard equipment on new automobiles. In that decade Michelin introduced the radial-ply tire, which is now standard for all automobiles in developed countries.
Pneumatic tires are usually retained on the wheel through the use of beads (hoops of wire) that are molded into the tire edges. The tire edges are placed in a shaped channel on the wheel rim’s circumference and are held firmly against the channel’s edges by the pressure exerted once the tire is inflated with air. Pressures range from about 30 pounds per square inch (200 kilopascals) for large, soft tires to approximately 100 pounds per square inch (700 kilopascals) for high-pressure, high-load applications.
The pressure is carried by layers of cords embedded in a rubber cover that in turn serves to contain the air, protect the cords, and give high friction with the ground. This felicitous combination is the reason why pneumatic tires are so widely used for transportation. More than 200 million of them are manufactured for motor vehicles alone each year, 90 percent for automobiles and 10 percent for trucks.
Pneumatic tires are designed to meet five main goals: low rolling resistance, low vertical stiffness (to cushion the ride), high sliding friction in both wet and dry conditions, high longitudinal and lateral stiffness (to minimize sliding motions in the “contact patch” where the tread meets the road), and resistance to wear and damage such as cutting, puncturing, and abrasion. In order to achieve these goals, tire designers must choose appropriate combinations of materials and structures, such as those described below.
A pneumatic tire is reinforced by layers of relatively inextensible cords that hold the air pressure and restrict deformation and growth of the tire during use. To this end cord materials must have high stiffness, resistance to repeated flexing, high strength-to-weight ratio, and good adhesion to rubber. Tire cords have been made of cotton, rayon, nylon, polyester, and glass, but steel and polyaramid (an extremely hard and stiff synthetic fibre) are currently the dominant materials in use.
Various rubber compounds are used in different parts of the tire. The liner, which is intended to minimize the loss of air, is usually made of butyl rubber because that material has a low permeability to gas. Sidewalls, on the other hand, must resist scraping, flexing, and attack by ozone in the air. A typical formulation for sidewalls (measured in parts by weight of each ingredient) would be 50 parts natural rubber (for resistance to heat buildup), 50 parts butadiene rubber (for abrasion resistance), and 50 parts carbon black (for reinforcement), along with small amounts of processing oil, antioxidant, and protective wax.
A tire’s treads must be especially resistant to abrasion. A tread compound might have no natural rubber at all but rather 65 parts styrene-butadiene rubber (for hardness and abrasion resistance), 35 parts butadiene rubber, and as much as 65 parts carbon black.
The addition of carbon black or silica as reinforcing agents in rubber compounds has greatly prolonged tire life. Also contributing to longevity are the abrasion-resistant polymer polybutadiene and improved vulcanization systems. (Vulcanization is the interlinking of rubber molecules in order to create a usable elastic solid. It is conducted by heating the rubber mix along with sulfur and various additives in a tire mold for about 20 minutes at a temperature of approximately 300° F, or 150° C.)
These advances, together with the almost universal adoption of radial-ply construction, have extended the life expectancy of some tires to more than 90,000 miles (150,000 km). Still, it must be emphasized that the formulation of rubber mixes for tires is more an art than a science. For instance, why a tread recipe based on butadiene rubber will have greater abrasion resistance than one based on natural rubber or styrene-butadiene rubber is not known.
The most important feature of tire design is the ply, i.e., the way in which the layers of reinforcing cords in the inner carcass are laid, or arranged. The three main types of arrangements are the bias-ply, the bias-ply belted, and the radial-ply belted.
In a bias-ply belted tire, another set of cords overlies the bias-laid ones. This extra set of cords, called a belt, is typically made of fibreglass. A radial-ply belted tire also has a belt running around the entire tire, but the cords are typically made of steel wire-mesh, hence the term “steel-belted radial” tire.
Belted bias-ply tires have a longer tread life and provide better gasoline mileage than conventional bias-ply tires. They also have better resistance to blowouts and reduce stopping distance in braking. Belted tires supplanted conventional bias-ply tires in the United States in the 1960s and ’70s. Meanwhile, though, radial-ply tires had already become standard equipment in Europe.
In the radial structure, layers of cords are arranged to lie almost perpendicular to the axis of the tire tube. Because the tire is actually a toroid (a circular tube) rather than a straight cylinder, the ply cords actually lie approximately 11 degrees off the perpendicular. This arrangement maximizes the tension across the width of the tire for a given air pressure, and hence gives greater resistance to lateral distortion of the tire on cornering.
Radial tires provide better steering characteristics and less rolling resistance than bias-ply tires, tend to run cooler, are safer in bad weather, and are said to give considerably more mileage. They are more expensive than bias-ply tires and have a slightly hard riding quality. Radial tires are now the most widely used type of tire. Bias-ply tires are still widely used in Third World countries, and they are also used for aircraft because they are less susceptible to damage by severe overloads.